Copper Mining Using Acidothiobacillus: Difference between revisions

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==Mechanism of Bacterial Leaching==
=Mechanism of Bacterial Leaching=
[[Image:At_ferrooxidans_EPS.jpg|thumb|350px|right|Figure 2. <i>At. ferrooxidans</i> binds to a metal sulfide substrate by way of an extracellular polymer matrix. The redox cycling conducted in the matrix helps to sustain the bacterium. (Valdés <i>et al.</i> 2008)]]
[[Image:At_ferrooxidans_EPS.jpg|thumb|350px|right|Figure 2. <i>At. ferrooxidans</i> binds to a metal sulfide substrate by way of an extracellular polymer matrix. The redox cycling conducted in the matrix helps to sustain the bacterium. (Valdés <i>et al.</i> 2008)]]
There has been much debate over how <i>At. ferrooxidans</i> contributes to the conversion of metal ores into soluble compounds. Although it has been proposed that the bacterium produces enzymes which directly mediate the reaction, studies have shown that iron is the only metal ion used to produce energy (Mignone and Donati 2004). Copper is not a metabolite of <i>At. ferrooxidans</i>, so the bacteria is not directly responsible for the solubilization of copper. Rather, it occurs through an abiotic reaction between the copper ores and the ferric ions and protons produced by the metabolic activities of the bacteria. These reactions regenerate the reduced iron and sulfur compounds that the bacteria reuptake to start the cycle anew.<br>
There has been much debate over how <i>At. ferrooxidans</i> contributes to the conversion of metal ores into soluble compounds. Although it has been proposed that the bacterium produces enzymes which directly mediate the reaction, studies have shown that iron is the only metal ion used to produce energy (Mignone and Donati 2004). Copper is not a metabolite of <i>At. ferrooxidans</i>, so the bacteria is not directly responsible for the solubilization of copper. Rather, it occurs through an abiotic reaction between the copper ores and the ferric ions and protons produced by the metabolic activities of the bacteria. These reactions regenerate the reduced iron and sulfur compounds that the bacteria reuptake to start the cycle anew.<br>
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<center>Cu<sub>3</sub>(OH)<sub>2</sub>(CO<sub>3</sub>)<sub>4</sub> (azurite) + 3H<sub>2</sub>O --> 3CuSO<sub>4</sub> + CO<sub>2</sub> + 4H<sub>2</sub>O</center><br>
<center>Cu<sub>3</sub>(OH)<sub>2</sub>(CO<sub>3</sub>)<sub>4</sub> (azurite) + 3H<sub>2</sub>O --> 3CuSO<sub>4</sub> + CO<sub>2</sub> + 4H<sub>2</sub>O</center><br>
This cycling of iron and sulfur maintains an energetically favorable situation for the bacteria; any ferric iron or sulfuric acid produced tends to undergo reduction with mineral ores to be subsequently re-oxidized by <i>At. ferrooxidans</i>. This cycling does not occur in a free-floating environment, however; the bacteria produce an extracellular biofilm made of polysaccharides and lipids which binds them to the mineral substrate and retains the iron and sulfur compounds as they go through the redox cycle (Fig. 2). The biofilm also helps to retain the bacterial cultures in heaps during irrigation (Sand and Gehrke 2006).
This cycling of iron and sulfur maintains an energetically favorable situation for the bacteria; any ferric iron or sulfuric acid produced tends to undergo reduction with mineral ores to be subsequently re-oxidized by <i>At. ferrooxidans</i>. This cycling does not occur in a free-floating environment, however; the bacteria produce an extracellular biofilm made of polysaccharides and lipids which binds them to the mineral substrate and retains the iron and sulfur compounds as they go through the redox cycle (Fig. 2). The biofilm also helps to retain the bacterial cultures in heaps during irrigation (Sand and Gehrke 2006).


==References==
==References==

Revision as of 00:45, 8 May 2011

Figure 1. A copper bioleaching plant in Uganda. (http://promine.gtk.fi/about/WorkPackage4.html)

Introduction

By Andrew Gipson


Copper has been one of mankind's most important metal resources since the beginning of civilization, and to this day it holds and important role in the functioning of modern society. It's main uses include piping, coinage, electronics, and even antibiotics (Wikipedia: Copper [1]). In 2009, global copper consumption topped 21 billion tons (Copper Development Association), and that kind of demand has led to the development of sophisticated mining techniques.
Copper is rarely found in its native form; most often it exists as ores made up of various copper sulfides and oxides that have little practical value of their own (Australian Atlas of Minerals, Resources, and Processing Centres). As such, methods for mining copper involve long and complex chemical processes to produce metal from the ores. This involves using harsh chemical reagents to pull the ore from the rock, and subsequent treatment to further isolate the metal and extract pure copper (Copper Mining Info [2]). These processes are energetically expensive, and they create a significant amount of pollutant chemicals such as sulfur dioxide and cyanide (Wikipedia: Copper Extraction Techniques [3]).
A viable alternative to these harmful procedures is biomining, or the use of bacterial metabolic processes to assist in the solubilization of metal ores. Acidothiobacillus ferrooxidans is a lithoautotroph famous for using ferrous iron (Fe2+) as an electron donor along with elemental sulfur. They grow in subterranean mineral deposits and even sewer pipes (Slonczewski and Foster 2011), and their activity has been linked to the processing of mineral ores (Rawlings 2002).

Copper Bioleaching Techniques

Mechanism of Bacterial Leaching

Figure 2. At. ferrooxidans binds to a metal sulfide substrate by way of an extracellular polymer matrix. The redox cycling conducted in the matrix helps to sustain the bacterium. (Valdés et al. 2008)

There has been much debate over how At. ferrooxidans contributes to the conversion of metal ores into soluble compounds. Although it has been proposed that the bacterium produces enzymes which directly mediate the reaction, studies have shown that iron is the only metal ion used to produce energy (Mignone and Donati 2004). Copper is not a metabolite of At. ferrooxidans, so the bacteria is not directly responsible for the solubilization of copper. Rather, it occurs through an abiotic reaction between the copper ores and the ferric ions and protons produced by the metabolic activities of the bacteria. These reactions regenerate the reduced iron and sulfur compounds that the bacteria reuptake to start the cycle anew.

Bacterial Metabolic Reactions

At. ferrooxidans has a diverse set of metabolic processes that allow it to be lithoautotrophic, but the most economically important activity this bacterium performs is the oxidation of ferrous iron (Fe2+) to ferric iron (Fe3+):

14Fe2+ + 3.5O2 + 14H+ --> 14Fe3+ + 7H2O


...and the oxidation of elemental sulfur (So) to sulfuric acid (H2SO4):

So + O2 + H2O --> H2SO3 + 1/2O2 --> H2SO4


These respiratory pathways produce the reactive species that initiate mineral solubilization (Leduc and Ferroni 1994).

Abiotic Solubilization and Regeneration of Bacterial Substrates

Copper sulfide ores are solubilized in a reaction where ferric iron complexed with sulfate attacks the copper compounds and becomes reduced to ferrous iron while generating copper sulfates and elemental sulfur (Hutchins et al. 1986):

Cu2S (chalcocite), CuS (covellite), CuFeS2 (chalcopyrite), Cu5FeS4 (bornite) + Fe2(SO4)3 --> CuSO4 + FeSO4 + So [unbalanced equation]


Copper oxides like azurite are somewhat less susceptible to attack by ferric iron; however, they are highly soluble in the low-pH solution produced by the bacterial production of sulfuric acid (Iasillo 1999):

Cu3(OH)2(CO3)4 (azurite) + 3H2O --> 3CuSO4 + CO2 + 4H2O


This cycling of iron and sulfur maintains an energetically favorable situation for the bacteria; any ferric iron or sulfuric acid produced tends to undergo reduction with mineral ores to be subsequently re-oxidized by At. ferrooxidans. This cycling does not occur in a free-floating environment, however; the bacteria produce an extracellular biofilm made of polysaccharides and lipids which binds them to the mineral substrate and retains the iron and sulfur compounds as they go through the redox cycle (Fig. 2). The biofilm also helps to retain the bacterial cultures in heaps during irrigation (Sand and Gehrke 2006).

References

Australian Atlas of Minerals, Resources, and Processing Centres. [4]
Biomining. [5]
"Copper." Wikipedia.[6]
Copper Development Association [7]
"Copper Extraction Techniques." Wikipedia. [8]
Copper Mining Info [9]
Iasillo E (1999) Metallurgical test program for copper leaching projects. In Copper Leaching, Extraction, and Electrowinning Techniques, ed. Jergensen GV. 124-128. Littleton: SME.
Leduc LG and Ferroni GD (1994) The chemolithotrophic bacterium Thiobacillus ferrooxidans. FEMS Microbiology Reviews 14:103-120.
Mignone CF and Donati ER (2004)ATP requirements for growth and maintenance of iron-oxidizing bacteria. Biochemical Engineering Journal 18: 211–216.
Rawlings DE (2002) Heavy metal mining using microbes. Annu. Rev. Microbiol. 56: 65-91.
Sand W and Gehrke T (2006) Extracellular polymeric substances mediate bioleaching/biocorrosion via interfacial processes involving iron(III) ions and acidophilic bacteria. Research in Microbiology 157: 49-56.
Slonczewski JL, Foster JW (2011) Microbiology: An Evolving Science. 2nd Ed. New York: W.W. Norton & Company. 1097 p.
Valdés J, Pedroso I, Quatrini R, Dodson RJ,Tettelin H, Blake R, Eisen JA, Holmes DS (2008) Acidithiobacillus ferrooxidans metabolism: from genome sequence to industrial applications. BMC Genomics 9: 597.

Edited by student of Joan Slonczewski for BIOL 238 Microbiology, 2011, Kenyon College.